In the manufacture of refrigeration systems containing a gaseous refrigerant it is necessary to monitor the equipment for leaks. Since it is desired that the systems be used for years without replenishment of refrigerant, small leaks should be detected and repaired. It is customary to establish a limit leak value, such as 0.4 ounces per year, and to ignore smaller leaks and detect and repair larger leaks.
Detecting the absolute value of gas concentration near a suspected leak location can give misleading results. Because the room in which testing takes place may have significant background levels of the test gas, it has become a common practice to detect the background level and subtract it from the measured concentration to indicate the levels above background. Further, it is known to frequently update the background level during leak detection to accommodate changes in the background. This method, called autozeroing, is subject to error when the detector is held near a leak long enough for the leak itself to contribute to the measured background, thereby greatly reducing sensitivity of the device.
An autozeroing arrangement is disclosed, for example, in the U.S. Pat. No. 3,786,675 to Delatorre et al which uses a gas detector which is subject to considerable temperature drift. Autozeroing is used to null the detector output to cancel out the temperature drift as well as the background. The patent proposes to hold the autozero value in memory for 12 seconds once a certain output is reached to allow measurements above the memory value. After that time, or upon actuating a switch, the circuit is again zeroed to cancel out background gas and reduce the effects of temperature drift. This approach could mask serious detector drift problems or sensitivity changes which should come to the attention of the operator, and would also reduce sensitivity by cancelling out gas readings due to a leak in addition to actual background gas.